This invention relates to a coil device, and more particularly to improvements in an induction coil for use in a dynamo electric machine or a protection circuit against overvoltages in a semiconductor rectifier circuit.
In general, in the rectifier circuit of the brushless exciter of an A.C. generator of large capacity, respective arms of semiconductor rectifier elements such as diodes or thyristors have heretofore been provided with snubber circuits in order to suppress commutation surges to the semiconductor rectifier elements and to partially allot the reverse voltage of the rectifier circuit, which is induced in the field coil of the generator during the asynchronous operation of the generator or the like, to the plus and minus arms. In this case, the induced field coil voltage, which is applied as the reverse voltage to the rectifier circuit when a field current is blocked by the semiconductor rectifier element, gives rise to a resonant phenomenon between the leakage inductance of the field coil and a capacitance existent in the rectifier circuit (or an associated circuit). Therefore, as the capacity of the generator becomes larger, the withstand voltage performance of the rectifier circuit must be enhanced. Accordingly, the snubber circuits which include voltage dividing capacitors for partially allotting the reverse voltage to the plus and minus arms have been indispensable as stated above.
The provision of such snubber circuits has led to the problem that, since the number of components increases, the reliability of the entire equipment degrades, and the problem that, since more enhanced performances are required of the respective elements of the snubber circuits with the increase of the capacity of the equipment, the price of the equipment rises, and besides, the size of the equipment becomes large inevitably.
Accordingly, the use of the snubber circuits in the rectifier circuit as described above is undesirable. In a case where, for example, a load connected to the rectifier circuit has a coil, the snubber circuits can be dispensed with in such a way that a low-pass filter for suppressing overvoltages is formed by utilizing the leakage inductance and distributed capacitance of the coil.
On the other hand, with the progress of semiconductor products in recent years, commutation surges and switching surges have become liable to frequently occur in the power source and load of a dynamo electric machine. Therefore, the dielectric strength requirements of the components of the dynamo electric machine have become more severe. By way of example, in the field circuit of the dynamo electric machine, a field circuit breaker is operated during the detection of a fault such as a short-circuit, so as to disconnect the field circuit. On account of a surge voltage which develops with the generation of an arc current and the dissipation of energy in a field coil when the circuit breaker has operated, excess withstand voltages are required of circuit elements. For this reason, an arrester such as a selenium arrester for overvoltages has been used.
In addition, the dynamo electric machine involves a high frequency voltage generated within itself. It has therefore been necessary to separately dispose an L-C low-pass filter as means for suppressing the high frequency voltage. The provision of the arrester and the low-pass filter circuit in the dynamo electric machine has led to the large size of the whole equipment and the increased cost thereof.
Also in this case, when a substantial low-pass filter is formed by the inductance and distributed capacitance of the field coil in the field circuit, the surge voltage and the high frequency voltage can be suppressed.
Besides, various coil devices for overcurrent suppression have been used, such as an A.C. reactor for suppressing fault currents or mutual interference between common voltage sources, an induction reactor assembled in a thyristor Leonard system, and an arc extinguishing reactor. Particularly in recent installations of large sizes and large capacities, the prior art coil devices have little economic merits.
Meanwhile, U.S. Pat. No. 3,686,523 issued to D. I. Gorden et al. on Aug. 22, 1972 teaches a technique wherein a flexible capacitive tape is disposed between the coil terminals of a dynomo electric machine or between the terminal and neutral point thereof so as to protect a rectifier device from overvoltages. This measure does not comprise improvements in the coil itself, and has disadvantages similar to those described above.